File:Anvil shaped cumulus panorama edit crop.jpg|thumb|350px|Cumulonimbus clouds often form thunderstorms.
'''Thunder''' is the sound caused by lightning
. Depending on the distance and nature of the lightning, thunder can range from a sharp, loud crack to a long, low rumble (brontide). The sudden increase in pressure
from lightning produces rapid Thermal expansion|expansion of the air
surrounding and within a bolt of lightning. In turn, this expansion of air creates a sonic shock wave, similar to a sonic boom
, which produces the sound of thunder, often referred to as a ''clap'', ''crack'', or ''peal of thunder''.